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Diagnose Exhaust Smoke

by Larry Carley copyright AA1Car.com

Exhaust smoke means trouble! A gasoline engine in good running condition should not produce any visible smoke in its exhaust. Steam is normal, and may appear to be white smoke on a cold morning due to condensation. But any other type of smoke in the exhaust means something is wrong.

Types of Exhaust Smoke:

>BLUE SMOKE: Bad news because it means the engine is burning oil. The underlying cause is usually worn valve guide seals or guides, or it may be worn or broken piston rings, or worn or damaged cylinders. Oil burning can eventually contaminate the catalytic converter and oxygen sensor. It also increases the risk of the engine running low on oil and losing oil pressure.

WHITE SMOKE: More bad news because it means the engine is burning coolant or transmission fluid. If the white smoke is coolant, the cause is likely a leaky head gasket or a crack in the cylinder head. If the white exhaust smoke is transmission fluid, the engine is sucking transmission fluid through a vacuum hose to the transmission.

If you have a diesel, white smoke may be unburned fuel passing through the engine, or it may be coolant. Some white smoke is normal following a cold start, especially during extremedly cold weather. But if you see continuous white smoke while driving, you have a problem: incorrect injector timing or a loss of compression in one or more cylinders.

BLACK SMOKE: Black smoke is normal for older diesel engines when they are first started or when they are accelerating hard, but there should be no black smoke is the vehicle is a late model car or truck with a "clean diesel" engine (electronic controls and high pressure common rail injectors).

Black smoke coming out of the tailpipe on a vehicle with a gasoline engine is a sign of a rich fuel mixture (too much fuel, not enough air). A brief puff of black smoke may be visible during hard acceleration when the fuel mixture goes momentarily rich, but under normal driving conditions and idling the exhasut should be transparent. If black smoke is visible all the time and the inside of the tailpipe is coated with heavy black carbon deposits, it would tell you the engine has a rich fuel condition.

On an older vehicle with a carburetor, a rich fuel condition and black smoke can be caused by a stuck or misadjusted automatic choke, a leaky metal or hollow plastic float inside the carburfetor fuel bowl, or a fuel saturated foam plastic float, or an incorrectly adjusted float (set too low). A restricted fuel filter may also contribute to a rich fuel mixture.

On a newer vehicle with fuel injection, black exhaust smoke can be caused by one or more leaky fuel injectors, too much fuel pressure (sticking fuel pressure regulator), a faulty MAF sensor or Oxygen sensor, or a engine computer fault.

with diesel engines, black smoke in the exhaust can occur during hard acceleration. But if the engine continues to produce black smoke, the engine is getting too much fuel. The underlying cause may be incorrect injector timing or an injection control problem.

How to Diagnose Exhaust Smoke:

BLUE SMOKE: Usually smells like burned toast. Check the oil level on the dipstick to see if the engine is using oil and dthe oil level is low (add oil as needed to bring it back up to the full mark. DO NOT let the engine oil level get too low or serious engine damage will result!. You can also do a compression check or leak down test to diagnose worn pistons or rings.

WHITE SMOKE: The exhaust will have a slightly sweet smell if the exhaust contains coolant, or a burned oil smell if it contains transmission fluid. Check the coolant level and the transmission fluid level. If the coolant is low and/or the engine has been overheating, pressure test the cooling system to see if it holds pressure. If it does not, the head gasket is probably leaking and needs to be replaced. If only the transmission fluid level is low, add the required type of transmission fluid to bring it back up to the full mark, and inspect the vacuum hose from the transmission for fluid inside. If it is passing fluid, replace the vacuum modulator valve on the transmission.

BLACK SMOKE: Check the adjustment and operation of the automatic choke if the engine is an older one with a carburetor. Check fuel pressure if the engine is a newer one with fuel injection. Also inspect the air filter, and check the OBD system for any sensor fault codes. The OBD sysem should detect a rich fuel condition, set a fault code (such as P0172 or P0175) and turn on the Check Engine light.


Repairs will depend on what is causing the exhaust smoke. If the engine is burning oil (blue smoke), it will probably need valve guide seals and/or piston rings, which means overhauling the engine.

If the engine is leaking coolant internally (white smoke), the cylinder head will have to come off to replace the head gasket. A less expesive repair alternative you might try is to add a bottle of cooling system head gasket sealer to the cooling system and hope that seals the leak (at least temporarily). Many of these products work quite well and can buy you valuable time so you can decide if you want to spend the money on repairs or sell or trade your vehicle.

If an older engine with a carburetor is blowing black smoke, the carburetor will need work. If adjustments to the automatic choke or float cannot restore a normal air/fuel mixture, the carburetor or choke will have to be rebuilt or replaced.

If a newer vehicle with fuel injection is running rich and passing black smoke, plug a scan tool into the OBD diagnostic connector and look for any fuel or sensor related fault codes. If your scan tool can also display fuel trim data, look at short and long term fuel trim (STFT and LTFT). If the fuel trim numbers are more than negative -8, it confirms the engine is running rich.

Diagnosis will require testing fuel pressure and the fuel pressure regulator to see if they are operating within specifications. If normal, the next step is to figure out which injector(s) are leaking. You can remove the spark plugs and look for ones that are carbon fouled (black deposits). That will tell you which cylinders are running rich and which injectors are the problem injectors.

engine smoke More Engine Diagnosis Articles:

Causes of High Oil Consumption

Valve Guide Repairs

Engine Compression Testing

Engine Leakdown Testing

Piston Rings (steel)

Piston Rings (stock & performance)

Piston Ring End Gap Recommendations

Measuring Blowby

Why Head Gaskets Fail

Head Gasket Failure: Common Causes

Preventing Repeat Head Gasket Failures

How To Find & Fix Coolant Leaks

Understanding Fuel Delivery Systems

How To Diagnose & Repair Carburetor Problems

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